1. Field of the Invention
The present invention relates to sound generation systems and more particularly concerns loudspeaker systems of very low frequency and high efficiency.
2. Description of Related Art
Loudspeaker systems are often provided with speaker components specifically adapted for operating at different frequency ranges, including low range, mid range and high range. Low range components often include special sub woofer speaker systems operable solely in the lowest frequency ranges, in the order of between about 30 and 100 hertz. Generally such very low sub woofer systems require high power driving signals so that an amplifier having a high power output at the low frequencies is needed to efficiently drive the sub woofer. Further, as frequency goes lower, the human ear has less sensitivity and even greater power is required for proper driving of the very low frequency speakers.
Particularly, for very large sound generation systems, such as those used in public address systems or other commercial applications to broadcast sound over very large areas, economic and other constraints will limit available power and may undesirably restrict low frequency output. Accordingly, efficiency of such sound generation systems at very low frequencies is an important consideration.
A common loudspeaker has a vibratory speaker cone, generally driven by a moving voice coil, with the cone having two faces, a forward or front face and a rearward or back face, which are driven as a unit to produce opposite phase sound waves. Particularly at low frequencies, sound waves produced at the rear face of the cone can interfere with the sound waves produced at the front face of the cone so that the net sound produced by the speaker is significantly diminished by destructive interference. At least partly for this reason speakers employed at low frequencies are placed in enclosures or provided with so-called "infinite baffle" arrangements to isolate sound produced from the rear face of the speaker cone from sound produced at the forward face of the speaker cone. This effectively eliminates one half of the sound output of the low frequency speaker, but prevents destructive interference. Effectively then, the output of the low frequency speaker can be reduced by 3 dB when used in most enclosures, thus greatly reducing efficiency. Lack of efficiency of large commercial type sound generation systems has been a widespread problem, requiring larger and more costly amplifying equipment and larger speaker enclosures.
Accordingly, it is an object of the present invention to provide low frequency system that avoids or minimizes above mentioned problems.